The temperature of the gas conveyed by a compressor rises according to the compression pressure ratio. In screw-type compressors which depend on the least possible play both between the two rotors and between the rotors and the casing, the thermal expansion caused on the parts of the compressor may lead to problems. It is known (DE-A-195 22 559) U.S. Pat. No. 5,924,855, by means of pre-admission, to lower the Temperature of the gas contained in the feed cells of the machine. By this is meant admitting cooler feed medium into the feed cells from a point of higher pressure. The pre-admission quantity supplied in each case to the chambers is small, as far as the efficiency of the machine is concerned. Thus, for example, when a screw-spindle machine is operated as a compressor (U.S. Pat. No. 4,812,110; U.S. Pat. No. 5,082,427), it is sufficient for only some of the conveyed gas to be recirculated for pre-admission. Also, when a screw-spindle machine is operated as a vacuum pump, it is necessary to comply with different preconditions from those occurring when it is operated as a compressor. Firstly, the pressure ratio is disproportionately higher in the vacuum mode than in the compressor mode, in particular typically well above 100. Secondly, in accordance with this pressure ratio, the temperature reached in the conveyed gas is substantially higher. Finally, it is necessary to ensure that the achievable vacuum is not impaired by pre-admission backflow.